In a radio communication system, in moving from a serving cell (source cell) to another cell, a mobile terminal performs switching processing of the serving cell called handover, and continues communication. In order to achieve handover of the mobile terminal, a base station that manages the source cell instructs the mobile terminal to transmit a measurement report when a predetermined event occurs. The predetermined event is, for example, deterioration of radio quality of the source cell. The measurement report generated by the mobile terminal includes measurement results of radio quality of the source cell and its neighboring cells. In response to receiving the measurement report from the mobile terminal, the base station of the source cell determines a cell (target cell) to which a radio link connection switches based on the measurement report, and initiates a handover procedure including signaling with the mobile terminal and the target cell.
Here, introduced is one of transmission events of the measurement report defined by 3GPP TS 36.331 V9.3.0 (2010-06), which is a technical specification regarding LTE (Long Term Evolution)/E-UTRAN (Evolved UTRAN). An essential portion of a reporting event defined as Event A3 (Neighbor becomes offset better than serving) in the above-described literature is expressed by the following Expression (1).PS+OS<PT+OT  (1)
PS in Expression (1) is a measurement result of radio quality of a source cell, and PT therein is a measurement result of radio quality of a neighboring cell. In a case of LTE, PS and PT are downlink RSRP (Reference Signal Received Power) or RSRQ (Reference Signal Received Quality). The RSRQ is a ratio of the RSRP to total received power (RSSI: Received Signal Strength Indicator).
OS in Expression (1) is an offset value that acts on radio quality of a downlink reference signal of the source cell, and is an HO parameter generally called an a3-offset (or hysteresis). Meanwhile, OT in Expression (1) is an offset value that acts on radio quality of a downlink reference signal of the neighboring cell, and is an HO parameter generally called a CIO (Cell Individual Offset). A value of the CIO (i.e., OT) may be set to be different for each neighboring cell. The CIO is included in a neighboring list (also called a neighboring cell list) of which the base station notifies mobile terminals connected to a cell managed by the base station itself.
When an operating condition of Expression (1) is set to the base station, the base station informs a mobile terminal, connected to the cell managed by the base station, about the operating condition of Expression (1). When a period in which the condition of Expression (1) is satisfied continues exceeding a predetermined period defined as a guard time (TTT: Time to Trigger), the mobile terminal transmits a measurement report to the base station that manages the source cell. In response to receiving the measurement report from the mobile terminal, the base station decides a target cell based on the measurement report, and initiates handover to the target cell.
However, when the initiation of the handover is too late or too early, a connection failure involving abnormal disconnection of a radio link (hereinafter referred to as RLF (Radio Link Failure)) occurs. In the present description, the connection failure involving RLF caused by inappropriate handover is called “handover failure”. Handover optimization or MRO (Mobility Robustness Optimization) is a technology of reducing handover failures by detecting them and adjusting HO parameters, and is one of major use cases of SON (Self-Organizing Network).
For example, Patent Literature 1 discloses a technology of controlling an initiation timing of handover by adjusting HO parameters, such as a3-offset, CIO and TTT in order to suppress handover failure and to increase a success rate of the handover. In addition, Non-Patent Literature 1 defines three types of handover failure involving RLF, i.e., “Too Late Handover”, “Too Early Handover”, and “Handover to Wrong Cell”, and discloses a detection technique thereof.
Too Late Handover corresponds to a situation where a mobile terminal that has experienced RLF in a source cell during execution of a handover procedure tries connection re-establishment (including re-establishment of a radio link) to a target cell, or a situation where a mobile terminal that has experienced RLF in a source cell before initiation of handover tries connection re-establishment to a cell different from the source cell.
Too Early Handover corresponds to a situation where a mobile terminal that has experienced RLF in a target cell during execution of a handover procedure or immediately after completion of handover tries connection re-establishment to a source cell.
Handover to Wrong Cell corresponds to a situation where a mobile terminal that has experienced RLF in a source cell or a target cell during execution of a handover procedure or immediately after completion of handover tries connection re-establishment to a cell different from both the source cell and the target cell.
Non-Patent Literature 1 discloses a technique in which the base station of the source cell detects a plurality of handover failure types by referring to information (i.e., a HANDOVER REPORT message and an RLF INDICATION message) transmitted/received between base stations through an inter-base-station interface (i.e., an X2 interface).
The RLF INDICATION message is transmitted from a base station B to a base station A, when a mobile terminal tries connection re-establishment to the base station B after RLF failure in the base station A. The RLF INDICATION message includes the following information:                Failure Cell ID: an identifier (Physical Cell Identity (PCI)) of a cell to which the mobile terminal had been connected before occurrence of connection failure;        Reestablishment Cell ID: an identifier (E-UTRAN Cell Global Identifier (ECGI)) of a cell to which the mobile terminal tried connection re-establishment; and        C-RNTI: an identifier (C-RNTI) of the mobile terminal in the cell to which the mobile terminal had connected before the occurrence of the connection failure.        
The HANDOVER REPORT message is generated regarding recently completed handover. The HANDOVER REPORT message is transmitted when handover failure is detected in a target cell immediately after a base station of the target cell transmits a handover completion notification (i.e., a UE Context Release message) to a base station of a source cell. The HANDOVER REPORT message includes the following information:                Type of detected handover failure (i.e., Too Early Handover or Handover to Wrong Cell);        Identifiers (ECGIs) of the source cell and the target cell;        Cell to which connection re-establishment was tried (in a case of Handover to Wrong Cell); and        Handover cause (a content sent from the base station of the source cell during handover preparation).        
As will be described below, Non-Patent Literature 1 discloses an outline of a technique of detecting the plurality of handover failure types (i.e., Too Late Handover, Too Early Handover, and Handover to Wrong Cell) by a base station of a source cell.
<Too Late Handover>
A base station of a source cell can detect Too Late Handover by receiving an RLF INDICATION message from a target cell (when handover is initiated) or a cell different from the source cell (when handover is not initiated).
<Too Early Handover>
A base station of a target cell can detect Too Early Handover by receiving, within a predetermined period (Tstore_UE_cntxt) after completion of incoming handover of a mobile terminal (i.e., after transmitting a UE CONTEXT RELEASE message), or while preparing the handover of the mobile terminal, an RLF INDICATION message regarding the mobile terminal from the source cell. In addition, the base station of the source cell can detect Too Early Handover by receiving, from the base station of the target cell, a HANDOVER REPORT message indicating Too Early Handover.
<Handover to Wrong Cell>
The base station of the target cell can detect Handover to Wrong Cell by receiving, within a predetermined period (Tstore_UE_cntxt) after completion of incoming handover of a mobile terminal (i.e., after transmitting a UE CONTEXT RELEASE message), or while preparing the handover of the mobile terminal, an RLF INDICATION message regarding the mobile terminal from a cell different from the source cell. In addition, the base station of the source cell can detect Handover to Wrong Cell by receiving, from the base station of the target cell, a HANDOVER REPORT message indicating Handover to Wrong Cell.
Alternatively, when receiving an attempt for connection re-establishment from a mobile terminal that has experienced RLF in a source cell during handover from the source cell to a target cell, a base station of a neighboring cell different from both the source cell and the target cell may transmit an RLF INDICATION message to the base station of the source cell. Consequently, the base station of the source cell can detect Handover to Wrong Cell by receiving the RLF
INDICATION message from the neighboring cell.